FinFET isolation structure and method for fabricating the same

ABSTRACT

A semiconductor device includes a semiconductor device and a semiconductor fin on the semiconductor substrate, in which the semiconductor fin has a fin isolation structure at a common boundary that is shared by the two cells. The fin isolation structure has an air gap extending from a top of the semiconductor fin to a stop layer on the semiconductor substrate. The air gap divides the semiconductor fin into two portions of the semiconductor fin. The fin isolation structure includes a dielectric cap layer capping a top of the air gap.

RELATED APPLICATIONS

This application is a continuation of U.S. Non-provisional patentapplication Ser. No. 15/414,413, filed on Jan. 24, 2017, which is acontinuation of U.S. Non-provisional patent application Ser. No.14/939,310, filed on Nov. 12, 2015. U.S. Non-provisional patentapplication Ser. No. 15/414,413 and U.S. Non-provisional patentapplication Ser. No. 14/939,310 are herein incorporated by reference.

BACKGROUND

When a semiconductor device such as a metal-oxide-semiconductorfield-effect transistor (MOSFET) is scaled down through varioustechnology nodes, device packing density and device performance arechallenged by device layout and isolation. In order to avoid leakagebetween neighboring devices (cells), the standard cell layout adoptsdummy polycrystalline silicon (poly) segments formed on edges of asilicon oxide definition (OD) region such as an active region of astandard cell, i.e., poly-on-OD-edge (PODE).

As the semiconductor IC industry has progressed into nanometertechnology process nodes in pursuit of higher device density, higherperformance, and lower costs, challenges from both fabrication anddesign have resulted in the development of three-dimensional (3D)devices such fin-like field effect transistors (FinFETs). Advantages ofFinFET devices include reducing the short channel effect and highercurrent flow. However, conventional FinFET devices and methods offabricating FinFET devices have not been entirely satisfactory inadopting the PODE for isolating two neighboring devices (cells).

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1A is a schematic three-dimensional diagram showing a semiconductordevice in accordance with some embodiments of the present disclosure.

FIG. 1B is a schematic top view of the semiconductor device shown inFIG. 1A.

FIG. 1C to FIG. 1F are schematic cross-sectional views showing varioustypes of fin isolation structure for the semiconductor device viewedalong line A1-A1′ in FIG. 1A.

FIG. 2A and FIG. 2B are schematic three-dimensional diagrams ofintermediate stages showing a method for fabricating a semiconductordevice in accordance with some embodiments of the present disclosure.

FIG. 2C to FIG. 2G are schematic cross-sectional views of intermediatestages showing a method for fabricating the semiconductor device viewedalong line B1-B1′ in FIG. 2A in accordance with some embodiments of thepresent disclosure.

FIG. 2F′ and FIG. 2G′ are schematic cross-sectional views ofintermediate stages showing a method for fabricating the semiconductordevice viewed along line B1-B1′ in FIG. 2A in accordance with certainembodiments of the present disclosure.

FIG. 3 is a flow chart showing a method for fabricating a semiconductordevice in accordance with some embodiments of the present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact.

Terms used herein are only used to describe the specific embodiments,which are not used to limit the claims appended herewith. For example,unless limited otherwise, the term “one” or “the” of the single form mayalso represent the plural form. In addition, the present disclosure mayrepeat reference numerals and/or letters in the various examples. Thisrepetition is for the purpose of simplicity and clarity and does not initself dictate a relationship between the various embodiments and/orconfigurations discussed. Further, spatially relative terms, such as“bottom”, “top” and the like, may be used herein for ease of descriptionto describe one element or feature's relationship to another element(s)or feature(s) as illustrated in the figures. The spatially relativeterms are intended to encompass different orientations of the device inuse or operation in addition to the orientation depicted in the figures.The device may be otherwise oriented (rotated 90 degrees or at otherorientations) and the spatially relative descriptors used herein maylikewise be interpreted accordingly.

Embodiments of the present disclosure are directed to fin-likefield-effect transistor (FinFET) devices between which a fin isolationstructure is disposed as a PODE for preventing leakage betweenneighboring devices (cells). The PODE helps to achieve better deviceperformance and better poly profile control. The fin isolation structurehas an air gap in a semiconductor fin to separate two neighboring cells,and a dielectric cap layer to cap a top of the air gap for subsequentprocesses such as metal landing. The air gap has a very low dielectricconstant, and is an excellent electrical isolator. Because the air gapis formed within the semiconductor fin, no additional area is needed toform the fin isolation structure, and thus the device size can beshrunk.

Referring to FIG. 1A and FIG. 1B, FIG. 1A is a schematicthree-dimensional diagram of a semiconductor device 100 in accordancewith some embodiments of the present disclosure, and FIG. 1B is aschematic top view of the semiconductor device 100 shown in FIG. 1A. Thesemiconductor device 100 includes a semiconductor substrate 110, a stoplayer 112 on the semiconductor substrate 110, a semiconductor fin 120 onthe stop layer 112, gate structures 130 a and 130 b crossing over thesemiconductor fin 120, and dummy gate structures 140 a, 140 b and 140 ccrossing over the semiconductor fin 120. The semiconductor substrate 110is defined as any construction including semiconductor materials,including, but is not limited to, bulk silicon, a semiconductor wafer,or a silicon germanium substrate. Other semiconductor materialsincluding group III, group IV, and group V elements may also be used.The stop layer 112 includes, but is not limited to, SiGeO_(x), SiGe,SiO_(x), SiP or SiPO_(x), where x is greater than 0. The stop layer 112has a thickness in a range from about 1 nm to about 50 nm. Thesemiconductor fin 120 protrudes from the semiconductor substrate 110.For forming the semiconductor fin 120, a semiconductor layer may beformed on the semiconductor substrate 110 and is etched until the stoplayer 112 is exposed. Because the etching stops at the top of the stoplayer 112, a height of the semiconductor fin 120 is approximately equalto a thickness of the semiconductor layer, such that the thickness ofthe semiconductor layer can be well controlled. Consequently, the heightof the semiconductor fin 120, and thus the channel width of the FinFET(semiconductor device 100) can be well controlled in view of therequirements of circuit design, thereby obtaining good deviceperformance.

The gate structures 130 a and 130 b may be referred herein as functionalor operational gate structures. As shown in FIG. 1B, a cell A and a cellB abutting the cell A are disposed on the semiconductor fin 120. Thedummy gate structures 140 a and 140 b are used to cover and protect theends of the semiconductor fin 120 of the cell A during processing, andthe dummy gate structures 140 b and 140 c are used to cover and protectthe ends of the semiconductor fin 120 of the cell B during processing,thereby providing additional reliability during processing. That is, thedummy gate structures 140 a, 140 b and 140 c are not electricallyconnected as gates for FinFET devices, and have no function in thecircuit. Each of the dummy gate structures 140 a, 140 b and 140 c has afin isolation structure 150. The cell A is electrically isolated fromthe cell B by the fin isolation structure 150 of the dummy gatestructures 140 b which acts as a PODE for preventing leakage between thecell A and the cell B. In some embodiments, another cell may beconnected to the cell A through the dummy gate structure 140 a, andanother cell may be connected to the cell B through the dummy gatestructure 140 c.

It is noted that embodiments of the present disclosure are alsoapplicable to only the cell A or the cell B, i.e. to a semiconductor finwith only the cell A or the cell B, in which two opposite ends of thesemiconductor fin have the fin isolation structures respectively.

Because the dummy gate structures 140 a, 140 b and 140 c have the samestructure, the dummy gate structure 140 b is used herein as an examplefor explaining the details of the fin isolation structure 150. As shownin FIG. 1B, the semiconductor fin 120 at the dummy gate structure 140 bhas the fin isolation structure 150 at a common boundary that is sharedby the two cells A and B. Referring to FIG. 1C, FIG. 1C is a schematiccross-sectional view showing a type of the fin isolation structure 150for the semiconductor device 100 viewed along line A1-A1′ in FIG. 1A. Asshown in FIG. 1C, the fin isolation structure 150 has an air gap 152dividing the semiconductor fin 120 into two portions 120 a and 120 b ofthe semiconductor fin 120. The air gap 152 extends to the stop layer 112from tops of two portions 120 a and 120 b of the semiconductor fin 120.The air gap 152 has a very low dielectric constant and is an excellentelectrical isolator, such that leakage between the cell A and the cell Bcan be avoided with a small width of the air gap 152. In someembodiments, the two portions 120 a and 120 b of the semiconductor fin120 are spaced at a distance D1 (the width of the air gap 152) in arange from about 5 nm to about 50 nm, and the claimed scope of thepresent disclosure is not limited in this respect. Because the air gap152 is formed within the semiconductor fin 120, no additional area isneeded to form the fin isolation structure 150, and thus the device sizecan be shrunk.

The fin isolation structure 150 includes a dummy gate dielectric 142 aon the portion 120 a, a dummy gate dielectric 142 b on the portion 120b, a dummy gate spacer 144 a on the dummy gate dielectric 142 a, a dummygate spacer 144 b on the dummy gate dielectric 142 b, and a dielectriccap layer 154 sandwiched between the dummy gate spacer 144 a and thedummy gate spacer 144 b. The dielectric cap layer 154 can be used as asupport for subsequent processes such as metal landing. Only arelatively small thickness T1 of the dielectric cap layer 154 is neededas long the dielectric cap layer 154 is strong enough to support theelements to be implemented in the subsequent processes. Thus, thedielectric constant of the area between the two portions 120 a and 120 bof the semiconductor fin 120 can be kept as small as possible, becausethe air gap 152 occupies a large part of the area between the twoportions 120 a and 120 b. In some embodiments, the dielectric cap layer154 has the thickness T1 extending from top surfaces of the dummy gatespacer 144 a and 144 b towards the semiconductor substrate 110, in whichthe thickness T1 is smaller than a height P1 of each of the dummy gatespacers 144 a and 144 b. The dielectric cap layer 154 may includesilicon nitride (SiN), oxynitride, silicon carbon (SiC), siliconoxynitride (SiON), oxide, and the like. In addition, the top surface ofthe dielectric cap layer 154 may be flat and coplanar with the topsurfaces of the dummy gate spacers 144 a and 144 b, thereby facilitatingthe subsequent processes.

In some embodiments, each of the dummy gate spacers 144 a and 144 bincludes a dielectric material, such as silicon nitride, siliconcarbide, silicon oxynitride, other suitable materials, and/orcombinations, but embodiments of the present disclosure are not limitedthereto. In some embodiments, each of the dummy gate dielectrics 142 aand 142 b may be made of one or more suitable dielectric materials suchas silicon oxide, silicon nitride, low-k dielectrics such as carbondoped oxides, extremely low-k dielectrics such as porous carbon dopedsilicon dioxide, a polymer such as polyimide, the like, or a combinationthereof. In other embodiments, the dummy gate dielectric 142 includesdielectric materials having a high dielectric constant (k value), forexample, greater than 3.9. The materials may include silicon nitrides,oxynitrides, metal oxides such as HfO₂, HfZrO_(x), HfSiO_(x), HfTiO_(x),HfAlO_(x), the like, or combinations and multi-layers thereof.

The semiconductor device 100 may further include epitaxial layers 122 aand 122 b on the semiconductor substrate 110. The epitaxial layers 122 ais located at one side of the two portions 120 a and 120 b of thesemiconductor fin 120, and is a source/drain portion of the cell A. Theepitaxial layer 122 b is located at the other side of the two portions120 a and 120 b of the semiconductor fin 120, and is a source/drainportion of the cell B. The epitaxial layers 122 a and 122 b may be dopedby performing an implanting process to implant appropriate dopants tocomplement the dopants in the semiconductor fin 120. In someembodiments, the epitaxial layers 122 a and 122 b may be formed byforming recesses (not shown) in the semiconductor fin 120 andepitaxially growing material in the recesses. The epitaxial layers 122 aand 122 b may be doped either through an implantation method asdiscussed above, or else by in-situ doping as the material is grown. Thesemiconductor device 100 may further include dielectric layers 146 a and146 b respectively on the epitaxial layers 122 a and 122 b, in which thedielectric layers 146 a and 146 b sandwich the dummy gate spacers 144 aand 144 b and the dielectric cap layer 154. The dielectric layers 146 aand 146 b may include silicon nitride (SiN), silicon carbon (SiC),silicon oxynitride (SiON), oxide, and the like.

Embodiments of the present disclosure further provide several types offin isolation structure 150 hereinafter. Referring to FIG. 1D, FIG. 1Dis a schematic cross-sectional view showing another type of finisolation structure 150 for the semiconductor device 100 viewed alongline A1-A1′ in FIG. 1A. As shown in FIG. 1D, the fin isolation structure150 has a first air gap 152 a on the stop layer 112, and a second airgap 152 b above the first air gap 152 a. The first air gap 152 a has atrapezoidal cross-section, and the second air gap 152 b has arectangular cross-section, i.e. a width D2 of a bottom of the first airgap 152 a is greater than a width D1 of a bottom of the second air gap152 b. The ratio of the height H2 of the second air gap 152 b and thedielectric cap layer 154 to the total height H1 of the first air gap 152a, the second air gap 152 b and the dielectric cap layer 154 is in arange from about 0.05 to about 1. The first air gap 152 a and the secondair gap 152 b divide the semiconductor fin 120 into two portions 120 aand 120 b of the semiconductor fin 120. The first air gap 152 a, thesecond air gap 152 b and the dielectric cap layer 154 have lowdielectric constants and are excellent electrical isolators, such thatleakage between the cell A and the cell B can be avoided with smallwidths of the first air gap 152 a, the second air gap 152 b and thedielectric cap layer 154. In some embodiments, the two portions 120 aand 120 b of the semiconductor fin 120 are spaced at a distance D1 (thewidth of the dielectric cap layer 154) in a range from about 5 nm toabout 50 nm, and the claimed scope of the present disclosure is notlimited in this respect. Because the first air gap 152 a, the second airgap 152 b and the dielectric cap layer 154 are formed within thesemiconductor fin 120, no additional area is needed to form the finisolation structure 150, and thus the device size can be shrunk.

Referring to FIG. 1E, FIG. 1E is a schematic cross-sectional viewshowing another type of fin isolation structure 150 for thesemiconductor device 100 viewed along line A1-A1′ in FIG. 1A. As shownin FIG. 1E, the fin isolation structure 150 has an air gap 152 cextending into a portion of the semiconductor substrate 110 through thestop layer 112 for a depth L1. The ratio of the depth L1 to the heightH3 of the air gap 152 c and the dielectric cap layer 154 is in a rangefrom about 0.05 to about 1. The air gap 152 c divides the semiconductorfin 120 into two portions 120 a and 120 b of the semiconductor fin 120.The air gap 152 c and the dielectric cap layer 154 have low dielectricconstants and are excellent electrical isolators, such that leakagebetween the cell A and the cell B can be avoided with small widths ofthe air gap 152 c and the dielectric cap layer 154. In some embodiments,the two portions 120 a and 120 b of the semiconductor fin 120 are spacedat a distance D1 (the width of the dielectric cap layer 154) in a rangefrom about 5 nm to about 50 nm, and the claimed scope of the presentdisclosure is not limited in this respect. Because the air gap 152 c andthe dielectric cap layer 154 are formed within the semiconductor fin120, no additional area is needed to form the fin isolation structure150, and thus the device size can be shrunk.

Referring to FIG. 1F, FIG. 1F is a schematic cross-sectional viewshowing another type of fin isolation structure 150 for thesemiconductor device 100 viewed along line A1-A1′ in FIG. 1A. As shownin FIG. 1F, the fin isolation structure 150 has an air gap 152 dextending into a portion of the semiconductor substrate 110 through thestop layer 112 for a depth L2. The air gap 152 d has a flat bottomenclosed by an arc surface, in which a width D3 of the flat bottom ofthe air gap 152 d is smaller than a width D1 of the dielectric cap layer154. The ratio of the depth L2 to the height H4 of the air gap 152 d andthe dielectric cap layer 154 is in a range from about 0.01 to about 1.The air gap 152 d divides the semiconductor fin 120 into two portions120 a and 120 b of the semiconductor fin 120. The air gap 152 d and thedielectric cap layer 154 have low dielectric constants and are excellentelectrical isolators, such that leakage between the cell A and the cellB can be avoided with small widths of the air gap 152 d and thedielectric cap layer 154. In some embodiments, the two portions 120 aand 120 b of the semiconductor fin 120 are spaced at a distance D1 (thewidth of the dielectric cap layer 154) in a range from about 5 nm toabout 50 nm, and the claimed scope of the present disclosure is notlimited in this respect. Because the air gap 152 d and the dielectriccap layer 154 are formed within the semiconductor fin 120, no additionalarea is needed to form the fin isolation structure 150, and thus thedevice size can be shrunk.

Referring to FIG. 2A to FIG. 2G, FIG. 2A and FIG. 2B are schematicthree-dimensional diagrams of intermediate stages showing a method forfabricating a semiconductor device 200 in accordance with someembodiments of the present disclosure, and FIG. 2C to FIG. 2G areschematic cross-sectional views of intermediate stages showing a methodfor fabricating the semiconductor device 200 viewed along line B1-B1′ inFIG. 2A in accordance with some embodiments of the present disclosure.

As shown in FIG. 2A, a semiconductor substrate 210 is provided, and astop layer 212 is formed on the semiconductor substrate 210 by, forexample, implantation or atomic layer deposition (ALD). Then, a Si layer(not shown) is epitaxially grown on the stop layer 212, and is patternedand etched using a photolithography technique to form a semiconductorfin 220. The semiconductor substrate 210 is defined as any constructionincluding semiconductor materials, including, but is not limited to,bulk silicon, a semiconductor wafer, or a silicon germanium substrate.Other semiconductor materials including group III, group IV, and group Velements may also be used. The stop layer 212 is formed from, but is notlimited to, SiGeO_(x), SiGe, SiO_(x), SiP or SiPO_(x), where x isgreater than 0. The stop layer 212 has a thickness in a range from about1 nm to about 50 nm. In some embodiments, a layer of photoresistmaterial (not shown) is deposited over the Si layer, and is irradiated(exposed) in accordance with a desired pattern and developed to remove aportion of the photoresist material. The remaining photoresist materialprotects the underlying material from subsequent processing operation,such as etching. It should be noted that other masks, such as an oxideor silicon nitride mask, may also be used in the etching process. A maskmay be used to control the shape of the semiconductor fin 220 during theepitaxial growth process.

As shown in FIG. 2B, gate structures 230 a, 230 b, 230 c, 230 d and 230c are formed to cross over the semiconductor fin 220, in which the gatestructures 230 b and 230 d are functional or operational gatestructures, and the gate structures 230 a, 230 c and 230 e will beprocessed later to become dummy gate structures. A cell A and a cell Babutting the cell A are defined on the semiconductor fin 220. The dummygate structures (gate structures 230 a and 230 c) are used to cover andprotect the ends of the semiconductor fin 220 of the cell A duringprocessing, and the dummy gate structures (gate structures 230 c and 230e) are used to cover and protect the ends of the semiconductor fin 220of the cell B during processing, thereby providing additionalreliability during processing. That is, the (dummy) gate structures 230a, 230 c and 230 e will be processed later to have no function in thecircuit. At this time, the gate structures 230 a, 230 b, 230 c, 230 dand 230 c have the same structures, and thus the gate structure 230 c isused herein as an example for explaining the details thereof.

As shown in FIG. 2C, a gate dielectric 242 is formed on thesemiconductor fin 220. The gate dielectric 242, which prevents electrondepletion, may include, for example, a high-k dielectric material suchas metal oxides, metal nitrides, metal silicates, transitionmetal-oxides, transition metal-nitrides, transition metal-silicates,oxynitrides of metals, metal aluminates, zirconium silicate, zirconiumaluminate, or combinations thereof. Some embodiments may include hafniumoxide (HfO₂) hafnium silicon oxide (HfSiO), hafnium silicon oxynitride(HfSiON), hafnium tantalum oxide (HfTaO), hafnium titanium oxide(HfTiO), hafnium zirconium oxide (HfZrO), lanthanum oxide (LaO),zirconium oxide (ZrO), titanium oxide (TiO), tantalum oxide (Ta₂O₅),yttrium oxide (Y₂O₃), strontium titanium oxide (SrTiO₃, STO), bariumtitanium oxide (BaTiO₃, BTO), barium zirconium oxide (BaZrO), hafniumlanthanum oxide (HfLaO), lanthanum silicon oxide (LaSiO), aluminumsilicon oxide (AlSiO), aluminum oxide (A1 ₂O₃), silicon nitride (Si₃N₄),oxynitrides (SiON), and combinations thereof. The gate dielectric 242may have a multilayer structure such as one layer of silicon oxide(e.g., interfacial layer) and another layer of high-k material. The gatedielectric 242 may be formed using chemical vapor deposition (CVD),physical vapor deposition (PVD), atomic layer deposition (ALD), thermaloxide, ozone oxidation, other suitable processes, or combinationsthereof.

As shown in FIG. 2C, a gate conductor 248 and a gate spacer 244 areformed on the gate dielectric 242, in which the gate spacer 244peripherally enclosing the gate conductor 248. The gate conductor 248may be formed from a conductive material and may be selected from agroup consisting of polycrystalline-silicon (poly-Si), poly-crystallinesilicon-germanium (poly-SiGe), metallic nitrides, metallic silicides,metallic oxides, metals, their combinations, and the like. Examples ofmetallic nitrides include tungsten nitride, molybdenum nitride, titaniumnitride, and tantalum nitride, or their combinations. Examples ofmetallic silicide include tungsten silicide, titanium silicide, cobaltsilicide, nickel silicide, platinum silicide, erbium silicide, or theircombinations. Examples of metallic oxides include ruthenium oxide,indium tin oxide, or their combinations. Examples of metal includetungsten, titanium, aluminum, copper, molybdenum, nickel, platinum, etc.The gate conductor 248 may be deposited by chemical vapor deposition(CVD), sputter deposition, or other techniques known and used in the artfor depositing conductive materials.

As shown in FIG. 2C, epitaxial layers 222 a and 222 b are formed on thestop layer 212. The epitaxial layers 222 a is formed at one side of thesemiconductor fin 220, and is a source/drain portion of the cell A. Theepitaxial layer 222 b is formed at the other side of the semiconductorfin 220, and is a source/drain portion of anther cell abutting the cellB. The epitaxial layers 222 a and 222 b may be doped by performing animplanting process to implant appropriate dopants to complement thedopants in the semiconductor fin 220. In some embodiments, the epitaxiallayers 222 a and 222 b may be formed by forming recesses (not shown) inthe semiconductor fin 120 and epitaxially growing material in therecesses. The epitaxial layers 222 a and 222 b may be doped eitherthrough an implantation method as discussed above, or else by in-situdoping as the material is grown. Dielectric layers 246 a and 246 b maybe formed respectively on the epitaxial layers 222 a and 222 b, in whichthe dielectric layers 246 a and 246 b sandwich the gate spacer 244. Thedielectric layers 246 a and 246 b may include silicon nitride (SiN),oxynitride, silicion carbon (SiC), silicon oxynitride (SiON), oxide, andthe like and may be formed by methods utilized to form such a layer,such as chemical vapor deposition (CVD), plasma enhanced CVD, sputter,and other methods known in the art.

Then, while the gate structures 230 b and 230 d are used as functionalor operational gate structures for the cell A and the cell B, the gatestructures 230 a, 230 c and 230 e are further processed in thesubsequent operations to become dummy gate structures each of which hasa fin isolation structure for isolating the cell A and the cell B.

As shown in FIG. 2D, a photoresist 250 is formed and patterned over thegate conductor 248, the gate spacer 244 and the dielectric layers 246 aand 246 b. In some embodiments, the photoresist 250 is formed bydepositing, exposing, and developing a layer of photoresist material.The photoresist 250 is patterned to expose the gate conductor 248. Thegate conductor 248 may be removed by suitable wet or dry etchingprocesses. For example, an etching solution such as, for example, NH₄OH,dilute-HF, and/or other suitable etchant may be used. Then, thephotoresist 250 is removed to obtain a structure as shown in FIG. 2E.

Thereafter, as shown in FIG. 2F, an exposed portion of the gatedielectric 242 and the underlying semiconductor fin 220 are etched toform an air gap (opening) 252 by using the gate spacer 244 as a mask.The gate dielectric 242 and the semiconductor fin 220 may be etchedusing reactive ion etch (RIE) and/or other suitable processes. Numerousother embodiments of methods to form the air gap (opening) 252 may besuitable. The air gap 252 divides the semiconductor fin 220 into twoportions 220 a and 220 b of the semiconductor fin 220 and ends on thestop layer 212. Then, as shown in FIG. 2G, a dielectric cap layer 254 isformed between the dummy gate spacer 244 a and the dummy gate spacer 244b. The dielectric cap layer 254 caps a top of the air gap 252, and isused as a support for subsequent processes such as metal landing. Only asmall thickness of the dielectric cap layer 254 is needed, as long thedielectric cap layer 254 is strong enough to support the elements to beimplemented in the subsequent processes. Thus, the dielectric constantof the area between the two portions 220 a and 220 b of thesemiconductor fin 220 can be kept as small as possible, because the airgap 252 occupies a large part of the area between the two portions 220 aand 220 b. The dielectric cap layer 254 may include silicon nitride(SiN), oxynitride, silicion carbon (SiC), silicon oxynitride (SiON),oxide, and the like and may be formed by methods utilized to form such alayer, such as chemical vapor deposition (CVD), plasma enhanced CVD,sputter, and other methods known in the art.

Referring to FIG. 2F′ and FIG. 2G′, FIG. 2F′ and FIG. 2G′ are schematiccross-sectional views of intermediate stages showing a method forfabricating the semiconductor device viewed along line B1-B1′ in FIG. 2Ain accordance with certain embodiments of the present disclosure. Incertain embodiments, the stop layer 212 underlying the semiconductor fin220 is also etched, such that the air gap 252 extends to a portion ofthe semiconductor substrate 210 through the stop layer 212 from tops oftwo portions 220 a and 220 b of the semiconductor fin 220. The largerdepth into the semiconductor substrate 210 can achieve higherperformance in terms of leakage current for the cells A and B. The stoplayer 212 may be etched by using C_(x)F_(y), NF_(x), N₂, O₂, Cl₂, Ar,SF_(x), C_(x)H_(y)F_(z) or HBr as an etchant, where x and y are greaterthan 0.

Then, as shown in FIG. 2G′, the dielectric cap layer 254 fills the airgap 252, and is used as a support for subsequent processes such as metallanding. Because the dielectric cap layer 254 and the air gap 252 havelow dielectric constants, and are excellent electrical isolators, suchthat leakage between the cell A and the cell B can be avoided even withsmall widths of the dielectric cap layer 254 and the air gap 252. Insome embodiments, the two portions 220 a and 220 b of the semiconductorfin 220 are spaced at a distance in a range from about 5 nm to about 50nm, and the claimed scope of the present disclosure is not limited inthis respect. Because the dielectric cap layer 254 and the air gap 252are formed within the semiconductor fin 220, no additional area isneeded to form the fin isolation structure, and thus the device size canbe shrunk.

It is noted that the air gap 252 may be formed with differentcross-sectional profiles. In some examples, the air gap 252 may includea first air gap on the stop layer 212, and a second air gap above thefirst air gap, in which the first air gap has a trapezoidalcross-section, and the second air gap has a rectangular cross-section,as shown in FIG. 1D. In certain examples, the air gap 252 may be formedwith a flat bottom enclosed by an arc surface, as shown in FIG. 1F.

Referring to FIG. 3 and FIG. 2A to FIG. 2F, FIG. 3 is a flow chartshowing a method for fabricating the semiconductor device 200 inaccordance with some embodiments of the present disclosure. The methodbegins at operation 306, where a stop layer 212 is formed on asemiconductor fin 220, as shown in FIG. 2A. Then, at operation 310, asemiconductor fin 220 is formed on the stop layer 212, as shown in FIG.2A. At operation 320, two cells A and B adjacent to each other areformed on the semiconductor fin 220, as shown in FIG. 2B. Gatestructures 230 a, 230 b, 230 c, 230 d and 230 c are formed to cross overthe semiconductor fin 220. The gate structure 230 b is a functional oroperational gate structure for the cell A, and the gate structures 230 aand 230 c will be processed in operations 350 and 360 to become dummygate structures acting as PODEs for protecting the ends of thesemiconductor fin 220 of the cell A during processing. The gatestructure 230 d is a functional or operational gate structure for thecell B, and the gate structures 230 c and 230 e will be processed inoperations 350 and 360 to become dummy gate structures acting as PODEsfor protecting the ends of the semiconductor fin 220 of the cell Bduring processing. The gate structure 230 c acts as the PODE forpreventing leakage between the cell A and the cell B.

At operation 330, a gate conductor 248 of the gate structure 230 c isformed on a top of the semiconductor fin 220 at a common boundary thatis shared by the two cells A and B, as shown in FIG. 2C. At operation340, a gate spacer 244 peripherally enclosing the gate conductor 248 isformed on the semiconductor fin 220, as shown in FIG. 2C. At operation350, the gate conductor 248 and the semiconductor fin 220 are etched toform a air gap 252, thereby dividing the semiconductor fin 220 into twoportions 220 a and 220 b of the semiconductor fin, as shown in FIG. 2Dto FIG. 2F. In some embodiments, the gate conductor 248, thesemiconductor fin 220, the stop layer 212 and a portion of thesemiconductor substrate 210 are etched to form a air gap 252, as shownin FIG. 2D and FIG. 2E′ to FIG. 2F′. At operation 360, a dielectric caplayer 254 is deposited into the air gap 252 to cap a top of the airgap242, as shown in FIG. 2G or FIG. 2G′. The dielectric cap layer 254may include silicon nitride (SiN), oxynitride, silicion carbon (SiC),silicon oxynitride (SiON), oxide, and the like and may be formed bymethods utilized to form such a layer, such as chemical vapor deposition(CVD), plasma enhanced CVD, sputter, and other methods known in the art.The dielectric cap layer 254 is used as a support for subsequentprocesses such as metal landing. The air gap 252 with the dielectric caplayer 254 is used for preventing leakage between the cell A and the cellB. At operation 370, an epitaxial layer 222 a or 222 b is formed at oneside of each of the two portions 220 a and 220 b of the semiconductorfin 220, as shown in FIG. 2G. The epitaxial layers 222 a is asource/drain portion of the cell A, and the epitaxial layer 222 b is asource/drain portion of anther cell abutting the cell B.

In accordance with an embodiment of the present disclosure, the presentdisclosure discloses a semiconductor device including a semiconductorsubstrate, a stop layer on the semiconductor substrate, a semiconductorfin on the stop layer, and two cells adjacent to each other on thesemiconductor fin, in which the semiconductor fin has a fin isolationstructure at a common boundary that is shared by the two cells. The finisolation structure has an air gap extending from a top of thesemiconductor fin to the stop layer, in which the air gap divides thesemiconductor fin into two portions of the semiconductor fin. The finisolation structure includes a dielectric cap layer capping a top of theair gap.

In accordance with another embodiment of the present disclosure, thepresent disclosure discloses a semiconductor device including asemiconductor substrate, a stop layer on the semiconductor substrate,and a semiconductor fin on the stop layer, in which each of two oppositeends of the semiconductor fin has a fin isolation structure. The finisolation structure has an air gap extending from a top of thesemiconductor fin to the stop layer, in which the air gap divides thesemiconductor fin into two portions of the semiconductor fin. The finisolation structure includes a dielectric cap layer capping a top of theair gap.

In accordance with yet another embodiment, the present disclosurediscloses a method for forming a semiconductor device. In this method, astop layer is formed on a semiconductor substrate, and a semiconductorfin is formed on the stop layer. Two cells adjacent to each other areformed on the semiconductor fin. A gate conductor is formed on a top ofthe semiconductor fin at a common boundary that is shared by the twocells. A gate spacer is formed to peripherally enclose the gateconductor. The gate conductor and the semiconductor fin are etched toform an air gap extending from a top of the semiconductor fin to thestop layer, thereby dividing the semiconductor fin into two portions ofthe semiconductor fin. A dielectric cap layer is deposited into the airgap to cap a top of the air gap.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A semiconductor device, comprising: a firstepitaxial layer; a first semiconductor layer having a first sidewallfacing the first epitaxial layer; a second semiconductor layer, wherein:an air gap is defined between a second sidewall of the firstsemiconductor layer and a first sidewall of the second semiconductorlayer, and the first sidewall of the first semiconductor layer isdiametrically opposite the second sidewall of the first semiconductorlayer; and a gate dielectric overlying the first semiconductor layer,wherein the air gap is further defined by a first sidewall of the gatedielectric.
 2. The semiconductor device of claim 1, comprising: adielectric cap layer defining a top of the air gap.
 3. The semiconductordevice of claim 2, comprising: a first gate spacer overlying the firstsemiconductor layer; and a second gate spacer overlying the secondsemiconductor layer, wherein the dielectric cap layer is disposedbetween the first gate spacer and the second gate spacer.
 4. Thesemiconductor device of claim 1, wherein: a second sidewall of the gatedielectric faces the first epitaxial layer, and the first sidewall ofthe gate dielectric is diametrically opposite the second sidewall of thegate dielectric.
 5. The semiconductor device of claim 4, wherein thesecond sidewall of the gate dielectric is in contact with the firstepitaxial layer.
 6. The semiconductor device of claim 1, wherein thefirst sidewall of the first semiconductor layer is in contact with thefirst epitaxial layer.
 7. The semiconductor device of claim 1,comprising: a stop layer underlying the first semiconductor layer anddefining a bottom of the air gap.
 8. The semiconductor device of claim7, comprising: a dielectric cap layer defining a top of the air gap,such that the air gap is between the stop layer and the dielectric caplayer.
 9. The semiconductor device of claim 1, comprising: a stop layerunderlying the first semiconductor layer, wherein the air gap is furtherdefined by a sidewall of the stop layer.
 10. The semiconductor device ofclaim 9, comprising: a substrate underlying the stop layer and defininga bottom of the air gap.
 11. The semiconductor device of claim 10,comprising: a dielectric cap layer defining a top of the air gap, suchthat the air gap is between the substrate and the dielectric cap layer.12. The semiconductor device of claim 10, wherein the air gap is furtherdefined by a sidewall of the substrate.
 13. The semiconductor device ofclaim 1, comprising: a substrate underlying the first semiconductorlayer and defining a bottom of the air gap.
 14. A method for forming asemiconductor device, the method comprising: forming a first gatestructure and a second gate structure over a semiconductor fin; forminga photoresist over the first gate structure, wherein the second gatestructure is exposed through the photoresist; removing the second gatestructure while the second gate structure is exposed through thephotoresist to define a first opening; and removing a portion of thesemiconductor fin through the first opening to define an air gap. 15.The method of claim 14, comprising: forming a first dielectric layerbetween the first gate structure and the second gate structure prior toforming the photoresist.
 16. The method of claim 14, comprising: forminga dielectric cap layer, the dielectric cap layer defining a top of theair gap.
 17. The method of claim 16, comprising: forming a first gatespacer and a second gate spacer prior to forming the photoresist,wherein: the second gate structure is between the first gate spacer andthe second gate spacer, and forming the dielectric cap layer comprisesforming the dielectric cap layer between the first gate spacer and thesecond gate spacer.
 18. The method of claim 14, wherein removing theportion of the semiconductor fin through the first opening comprisesdividing the semiconductor fin into a first portion and a second portionseparated from the first portion by the air gap.
 19. A semiconductordevice, comprising: a first semiconductor fin; a second semiconductorfin; a first gate spacer overlying the first semiconductor fin; a secondgate spacer overlying the second semiconductor fin; and a dielectric caplayer between the first gate spacer and the second gate spacer, whereinan air gap is defined between the first semiconductor fin and the secondsemiconductor fin and below the dielectric cap layer.
 20. Thesemiconductor device of claim 19, comprising: a gate dielectricoverlying the first semiconductor fin, wherein the air gap is furtherdefined by a first sidewall of the gate dielectric.